Bias current circuit and semiconductor integrated circuit

ABSTRACT

A bias current circuit controls an oscillator that generates an oscillation signal of a frequency corresponding to an input current. The circuit includes a part that detects fluctuation of a control current for variably controlling the frequency of the oscillation signal and a part that generates an input current in which a fluctuation component of the control current is canceled using a current for cancelling the detected fluctuation of the control current.

FIELD

Embodiments of the present invention relate to a bias current circuitand a semiconductor integrated circuit.

BACKGROUND

A phase-locked loop circuit (PLL) is used to generate a reference clocksignal in various electronic circuits. A PLL has an oscillator thatgenerates an oscillation signal of a frequency corresponding to anexternal signal. As one of various known oscillators, there is acurrent-controlled oscillator in which a frequency of an oscillationsignal is controlled by an input control current. The control current isgenerated by, for example, a current source capable of controlling anamount of a current, and a current mirror copying a current flowingthrough the current source.

However, there is a problem that when the power-supply voltage of theoscillator fluctuates, the control current also fluctuates. As a result,it is difficult to generate a precise oscillation signal.

SUMMARY

Embodiments provide a bias current circuit that controls an oscillatorto generate a precise oscillation signal therein, and a semiconductorintegrated circuit that includes the bias current circuit and theoscillator.

According to the embodiments, a bias current circuit is provided thatcontrols an oscillator that generates an oscillation signal of afrequency corresponding to an input current. The bias current circuitincludes a portion that detects fluctuation of a current for variablycontrolling the frequency of the oscillation signal, and a cancellingportion that generates an input current in which a fluctuation componentof the control current has been canceled using a current for cancellingout the detected fluctuation of the control current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of an all-digitalphase-locked loop (ADPLL) 100.

FIG. 2 is a circuit diagram of a semiconductor integrated circuit 20according to a first embodiment.

FIG. 3 is a diagram for explaining an operation of a bias currentcircuit 2.

FIG. 4 is a circuit diagram of a semiconductor integrated circuit 20 athat is a modification example of the embodiment shown in FIG. 2.

FIG. 5 is a circuit diagram of a semiconductor integrated circuit 20 baccording to a second embodiment.

DETAILED DESCRIPTION

In the following, embodiments are explained in detail with reference tothe drawings.

First Embodiment

FIG. 1 is a block diagram illustrating a configuration of an all-digitalphase-locked loop circuit, hereinafter an ADPLL 100. The ADPLL 100 isprovided with a digital filter 10, a semiconductor integrated circuit20, and a buffer 30.

The digital filter 10 generates a control signal DCNT corresponding to aphase difference between a reference signal CKref whose frequency isconstant with high precision, and an output signal CKout. The controlsignal DCNT is a digital signal, and it is changed to reduce the phasedifference between the reference signal CKref and the output signalCKout.

The semiconductor integrated circuit 20 includes an oscillator 1 and abias current circuit 2. The bias current circuit 2 generates a currentcorresponding to the control signal DCNT and inputs the current as aninput current Iin to the oscillator 1. The oscillator 1 is acurrent-controlled oscillator and it generates an oscillation signalCKosc having a frequency which is determined according to the inputcurrent Iin. That is, the semiconductor integrated circuit 20 includingan oscillator generates an oscillation signal CKosc at a frequencycorresponding to the control signal DCNT.

The buffer 30 amplifies the oscillation signal CKosc and outputs theamplified output signal as CKout. The output signal CKout is again inputas a feedback signal to the digital filter 10.

By a feedback control as described above, the ADPLL 100 is capable ofgenerating an output signal CKout of a constant frequency.

However, a power-supply voltage VDD supplied to the ADPLL 100 mayfluctuate due to some reasons. When the power-supply voltage VDDfluctuates, the input current Iin may also fluctuate, and the resultingfrequency of the oscillation signal CKosc will also fluctuate. As aresult, jitter in the output signal CKout may increase.

Therefore, in the present embodiment, a bias current circuit 2 in thesemiconductor integrated circuit 20 is provided, such that the inputcurrent Iin does not fluctuate even when the power-supply voltage VDDfluctuates.

FIG. 2 is a circuit diagram of the semiconductor integrated circuit 20according to the first embodiment. In the circuit illustrated in FIG. 2,the bias current circuit 2 is the portion that omits the oscillator 1from the semiconductor integrated circuit 20.

The bias current circuit 2 is provided with pMOS transistors Qp0-Qp2,nMOS transistors Qn0-Qn2, a capacitor C1, a resistor R1, and a currentsource I1.

The transistor Qp0 and the current source I1 are connected in seriesbetween a power-supply terminal (referred to as the power-supplyterminal, hereinafter “VDD”) that supplies a power-supply voltage(second reference voltage) VDD and a ground terminal (referred to as theground terminal “VSS” hereafter) that supplies a ground voltage (firstreference voltage) VSS.

The transistors Qp1, Qn0, and the oscillator 1 are also connected inseries between the power-supply terminal VDD and the ground terminalVSS. When the oscillator 1 includes two transistors that are connectedin series, a total of four transistors are connected in series betweenthe power-supply terminal VDD and the ground terminal VSS.

The transistor Qp0 is diode-connected. The drain and the gate of thetransistor Qp0 are short-circuited at node ND0 and are therefromconnected to the gate of the transistor Qp1. The transistor Qn0 is alsodiode-connected. The drain and the gate of the transistor Qn0 are alsoshort-circuited, here at node ND1.

The transistor Qn1 is connected in parallel with the oscillator 1. Acapacitor C1 is connected between the node ND1 and a node ND2 that iselectrically connected to the gate of the transistor Qn1. Thetransistors Qp2 and Qn2 are connected in series between the power-supplyterminal VDD and the ground terminal VSS. The gate of the transistor Qp2is connected to the gates of the transistors Qp0 and Qp1. The transistorQn2 is diode-connected. The drain and the gate of the transistor Qn2 areshort-circuited at node ND3. The resistor R1 is connected between thenode ND2 and the node ND3. It is designed such that transconductance ofthe transistor Qn0 and Qn1 is equal to each other.

The current flowing through the current source I1 is controlledaccording to the control signal DCNT. Furthermore, a current mirror iscomposed of the transistors Qp0 and Qp1. Therefore, a currentproportional to the current flowing through the current source I1, thatis, the current flowing between the source and the drain of thetransistor Qp0, flows between the source and the drain of the transistorQp1. The current flowing through the transistor Qp1 is referred to asthe control current Icnt in the following. The frequency of theoscillation signal CKosc is variably controlled by the control currentIcnt.

Due to fluctuation of the power-supply voltage VDD, the control currentIcnt may also fluctuate. Therefore, if the control current Icnt, as itmay fluctuate, is input to the oscillator 1, there is a risk that thefrequency of the output oscillation signal CKosc may also fluctuate.

Therefore, the transistors Qn0-Qn2, Qp2, the capacitor C1, and theresistor R1 are provided to suppress such fluctuation of the frequency.

The transistor Qn0 is an example of a conversion part. A set of thecapacitor C1 and the resistor R1 is an example of a filter part. Thetransistor Qn1 and a connection node of the transistors Qn0 and Qn1 arean example of a cancel part. Furthermore, a set of the transistor Qn0,the capacitor C1, and the resistor R1 is an example of a detection part.

The transistors Qp2 and Qn2 generate a bias current Ib for operating thetransistor Qn1. The gate of the transistor Qp2 is connected to the gateof the transistor Qp1. Therefore, the bias current Ib is proportional tothe control current Icnt. Due to the bias current Ib, a voltage of thenode ND3 is input to the gate of the transistor Qn1 via the resistor R1.When the control current Icnt does not fluctuate, the gate voltage ofthe transistor Qn1 is determined according to the bias current Ib, andthis voltage becomes an operation reference voltage of the transistorQn1.

The transistor Qn0 converts the control current Icnt to a voltage. Thatis, when the control current Icnt fluctuates, the voltage at the nodeND1 fluctuates.

Due to the capacitor C1 and the resistor R1, a high-pass filter (HPF)and a low-pass filter (LPF) are composed. When the voltage of the nodeND1 fluctuates and has a high-frequency component, the high-frequencycomponent (that is, a voltage fluctuation component) is extracted and istransmitted to the node ND2 via the capacitor C1. As a result, accordingto the voltage fluctuation of the node ND1, in other words, according tothe fluctuation of the control current Icnt, the above-described gatevoltage of the transistor Qn1 fluctuates. The cut-off frequencies of theHPF and the LPF may be designed by considering an anticipatedfluctuation of the power-supply voltage VDD.

As described above, in the present embodiment, the fluctuation of thecontrol current Icnt is detected as a voltage fluctuation of the node ND1 and this fluctuation is transmitted to the transistor Qn1 via thecapacitor C1. Since the number of devices involved in the transmissionis small, the fluctuation of the control current Icnt can be transmittedto the transistor Qn1 with high precision.

A current Ican corresponding to a gate voltage (voltage of the node ND2)flows between the source and the drain of the transistor Qn1. As can beseen from the above explanation, the current Ican fluctuates accordingto the voltage of the node ND1, in other words, according to thefluctuation of the control current Icnt.

A portion of the control current Icnt is input as the input current Iinat the input terminal of the oscillator 1, and a portion of the controlcurrent Icnt also flows as the cancelling current Ican between thesource and the drain of the transistor Qn1. Therefore, a current,obtained by subtracting the current Ican from the control current Icnt,is generated by the connection node of the transistors Qn0, Qn1, and isinput as the input current Iin (=Icnt−Ican) to the oscillator 1.

FIG. 3 is a diagram for explaining an operation of the bias currentcircuit 2. A vertical axis represents current values of the currentsIcnt, Iin, Ican, and a horizontal axis represents time t. When thepower-supply voltage VDD fluctuates, the control current Icnt alsofluctuates. Then, the current Ican is generated that fluctuates alongwith the fluctuation of the control current Icnt. Here, thetransconductance of the transistors Qn0 and Qn1 is equal to each other.Therefore, the fluctuation amplitude of the current Icnt is equal tothat of the current Ican.

The current Ican fluctuates in the same way, i.e., with the samefrequency and cancelling amplitude as the control current Icnt.Therefore, fluctuating portions of the currents are canceled and aconstant input current Iin is input to the oscillator. As a result, theinput current Iin (=Icnt−Ican) is constant even when the current Icntfluctuates.

In order to reduce power consumption of the bias current circuit 2, asize of the transistor Qp2 may be made smaller than that of thetransistor Qp1. For example, by making the size of the transistor Qp2 tobe 1/n (n is a number larger than 1) of the size of the transistor Qp1,the bias current Ib can be 1/n of the control current Icnt. In thiscase, the size of the transistor Qn1 may be n times of a size of thetransistor Qn0. This allows the transconductance of the transistor Qn0and Qn1 to be equal to each other.

A size of a transistor may be adjusted by both a gate length L and agate width W. However, when the gate length L is constant in amanufacturing process, the size of the transistor can be adjusted by thegate width W.

As described above, in the first embodiment, the fluctuation of thecontrol current Icnt is detected and the current Ican for cancelling thefluctuation of the control current Icnt is generated. Then, the inputcurrent Iin, obtained by subtracting the current Ican from the controlcurrent Icnt, is input to the oscillator 1. As a result, even when thecontrol current Icnt fluctuates, the oscillator 1 can generate theoscillation signal CKosc at a precise frequency.

FIG. 4 is a circuit diagram of a semiconductor integrated circuit 20 athat is a modification example of the embodiment shown in FIG. 2. In abias current circuit 2 a of FIG. 4, a variable resistor R2 is used inplace of the transistor Qn0 of FIG. 2. A resistance value of thevariable resistor R2 is designed to be an inverse of thetransconductance of the transistor Qn1. This allows the bias currentcircuit 2 a of FIG. 4 to operate in the same manner as the bias currentcircuit 2 of FIG. 2. When the control current Icnt flowing through thevariable resistor R2 is small, a voltage drop due to the variableresistor R2 is small. Therefore, the power-supply voltage VDD can belowered.

Second Embodiment

In the above-described first embodiment, the transistor Qn0 is providedbetween the transistor Qp1 and the oscillator 1 to detect thefluctuation of the control current Icnt. In contrast, in a secondembodiment described below, the fluctuation of the control current Icntis detected without providing a device between the transistor Qp1 andthe oscillator 1.

FIG. 5 is a circuit diagram of a semiconductor integrated circuit 20 baccording to the second embodiment. The following explains mainly thedifferences in the circuit as compared to the circuit of FIG. 2.

In contrast to the circuit of FIG. 2, in the bias current circuit 2 b ofFIG. 5, the drain of the transistor Qp1 is directly connected to theoscillator 1. For example, when the oscillator 1 includes twotransistors that are connected in series, a total of three transistorsare connected in series between the power-supply terminal VDD and theground terminal VSS. Therefore, as compared to FIG. 2, in the circuit ofFIG. 5, the number of devices connected between the power-supplyterminal VDD and the ground terminal VSS are smaller.

Furthermore, the bias current circuit 2 b is further provided with apMOS transistor Qp3 and an nMOS transistor Qn3 that are connected inseries between the power-supply terminal VDD and the ground terminalVSS. The gate of the transistor Qp3 is connected to the node ND0 that iselectrically connected to the gate of the transistors Qp0-Qp2. Thetransistor Qn3 is diode-connected. The drain and the gate of thetransistor Qn3 are short-circuited at node ND4. A capacitor C1 isconnected between the node ND4 and the node ND2. It is designed suchthat transconductance of the transistors Qn3 and Qn1 are equal to eachother.

The transistors Qp3 and Qn3 are an example of the conversion part.

Since the gate of the transistor Qp1 and the gate of the transistor Qp3are connected, a current proportional to the control current Icnt flowsbetween the source and the drain of the transistor Qp3. Therefore, whenthe control current Icnt fluctuates, the current flowing between thesource and the drain of the transistor Qp3 also fluctuates in proportionto the fluctuation of the control current Icnt. As a result, a currentflowing between a source and a drain of the transistor Qn3 alsofluctuates and a voltage of the node ND4 also fluctuates.

Thus, the voltage of the node ND4 also fluctuates based upon thefluctuation of Icnt. The voltage fluctuation of the node ND4 istransmitted to the node ND2 by the HPF and LPF that are composed of theresistor R1 and the capacitor C1.

Other operations are substantially the same as those of FIG. 2. As aresult, the input current Iin, obtained by subtracting from the controlcurrent Icnt, the cancel current Ican that corresponds to a fluctuationcomponent of the control current Icnt, is input to the oscillator 1.

In order to detect the fluctuation of the control current Icnt with highprecision, it is desirable that a size of the transistor Qp3 be aboutthe same as the size of the transistor Qp1, without changing the size ofthe transistor Qp3.

As described above, in the second embodiment, the fluctuation of thecontrol current Icnt is detected and the current Ican for cancelling thefluctuation of the control current Icnt is generated. Therefore, evenwhen the control current Icnt fluctuates, the oscillator 1 can generatethe oscillation signal CKosc of a precise frequency.

Furthermore, in the first embodiment, the transistors Qp1, Qn0, and theoscillator 1 are connected in series between the power-supply terminalVDD and the ground terminal VSS, whereas in the second embodiment, thetransistor Qp1 and the oscillator 1 are directly connected in seriesbetween the power-supply terminal VDD and the ground terminal VSS. Fewerdevices are connected between the power-supply terminal VDD and theground terminal VSS in the second embodiment. Therefore, the secondembodiment can be operated even at a lower power-supply voltage than thefirst embodiment.

In the first and the second embodiments, an example in which the biascurrent circuit 2 controls the oscillator 1 is described. However, thebias current circuit 2 can be generally used for controlling a circuitwith a current input.

The bias current circuit of FIG. 2 and the like is merely an example andvarious modifications are possible. For example, at least some of theMOS transistors may be configured by using other semiconductor devicessuch as bipolar transistors. Furthermore, it is also possible toconfigure the bias current circuit in such a manner that conductivitytypes of the transistors are reversed and, accordingly, the connectionpositions of the power-supply terminal and the ground terminal arereversed. In this case, the basic operation principle is also the same.

The entire circuit of the oscillator and the bias current circuitaccording to the present invention may be formed on a same semiconductorsubstrate; and a part of the circuit may also be formed on a separatesemiconductor substrate. Further, at least a part of the oscillator andthe bias current circuit according to the present invention may bemounted on a printed circuit board and the like by using a discretecomponent.

Several embodiments of the present invention are explained in the above.However, these embodiments are presented by way of example and are notintended to limit the scope of the invention. These embodiments can beembodied in various forms and various omissions, substitutions andmodifications can be performed within the scope without departing fromthe spirit of the invention. These embodiments and modificationsthereof, same as being within the scope and spirit of the invention, arewithin the scope of the invention as described in the appended claimsand its equivalents.

What is claimed is:
 1. A bias current circuit that controls anoscillator that generates an oscillation signal of a frequencycorresponding to an input current, comprising: a detection part thatdetects fluctuation of a control current for variably controlling thefrequency of the oscillation signal; and a cancelling part generating acancelling current for cancelling the detected fluctuation of thecontrol current to generate the input current to the oscillator, whereinthe detection part includes a conversion part that converts the controlcurrent to a voltage, and a filter part that extracts a high-frequencycomponent of the voltage and supplies the high-frequency component ofthe voltage to the cancelling part, and wherein the cancelling partgenerates the cancelling current based on the high-frequency componentof the voltage.
 2. The bias current circuit according to claim 1,wherein the conversion part includes a first MOS transistor thatincluding a first drain, a first source and a first gate, the controlcurrent flowing between the first source and the first drain, the firstdrain and the first gate being short-circuited, and the voltage beingoutput from the first drain, the cancelling part includes a second MOStransistor including a second drain, a second source and a second gate,the cancelling current flowing between the second source and the seconddrain, an output voltage of the filter part being input to the secondgate, and a first reference voltage being input to the second source,and the input current is obtained by subtracting the cancelling currentfrom the control current, the cancelling current flowing in a shortcircuit between the first source and the second drain.
 3. The biascurrent circuit according to claim 2, wherein transconductance of thefirst MOS transistor is equal to transconductance of the second MOStransistor.
 4. The bias current circuit according to claim 1, whereinthe conversion part includes a third MOS transistor including a thirddrain, a third source and a third gate, a current proportional to thecontrol current flowing between the third source and the third drain,and a fourth MOS transistor including a fourth drain, a fourth sourceand a fourth gate, the fourth drain being connected to the third drain,a first reference voltage being input to the fourth source, the fourthdrain and the fourth gate being short-circuited, and the voltage beingoutput from the fourth drain, and the cancelling part includes a fifthMOS transistor including a fifth drain, a fifth source and a fifth gate,the fifth drain being connected to the oscillator, an output voltage ofthe filter part being input to the fifth gate, and the first referencevoltage being input to the fifth source.
 5. The bias current circuitaccording to claim 4, wherein the transconductance of the fourth MOStransistor is equal to transconductance of the fifth MOS transistor. 6.The bias current circuit of claim 1, wherein the cancelling current hasthe same frequency as the high-frequency component of the voltage. 7.The bias current circuit of claim 6, wherein the cancelling currentamplitude is equal to the fluctuation amplitude of the control current.8. The bias current circuit of claim 6, wherein the output of theoscillator is proportional to a change in the control current.
 9. Asemiconductor integrated circuit, comprising: an oscillator thatgenerates an oscillation signal of a frequency corresponding to an inputcurrent; and a bias current circuit that controls the oscillator,wherein the bias current circuit includes a detection part that detectsfluctuation of a control current for variably controlling the frequencyof the oscillation signal, and a cancelling part that uses a cancellingcurrent for cancelling the detected fluctuation of the control currentto generate the input current that is obtained by cancelling afluctuation component of the control current, the detection partincludes a conversion part that converts the control current to avoltage, and a filter part that extracts a high-frequency component ofthe voltage and supplies the high-frequency component of the voltage tothe cancelling part, the cancelling part generates the cancellingcurrent based on a high-frequency component of the voltage, theconversion part includes a first MOS transistor that includes a firstdrain, a first source and a first gate, the control current flowingbetween the first source and the first drain, the first drain and thefirst gate being short-circuited, and the voltage being output from thefirst drain, the cancelling part includes a second MOS transistorincluding a second drain, a second source and a second gate, thecancelling current flowing between the second source and the seconddrain, an output voltage of the filter part being input to the secondgate, and a first reference voltage being input to the second source,the input current is obtained by subtracting the cancelling current fromthe control current, where the cancelling current flows in ashort-circuit between the first source and the second drain, and thetransconductance of the first MOS transistor is equal totransconductance of the second MOS transistor.
 10. A bias currentcircuit configured to control an oscillator that generates anoscillation signal corresponding to a change in an input current to thecircuit, comprising: a first MOS transistor configured to receive theinput current at the drain thereon, wherein the drain and the gatethereof are short-circuited and further connected to a load; a secondMOS transistor having a gate coupled to the gate of the first MOStransistor, the drain thereof configured to receive the input current;the drain of the second MOS transistor coupled to the input to anoscillator; and a compensation circuit coupled between the input currentand a load, wherein the compensation circuit generates compensationcurrent at the same frequency as, and proportional in amplitude to, thechanges in the input current.
 11. The bias current circuit of claim 10,wherein the compensation circuit includes a third MOS transistorconnected in series with a fourth MOS transistor, wherein the gate ofthe third MOS transistor is connected to the gate of the first MOStransistor and gate of the fourth MOS transistor is short-circuited tothe drain thereof and connected to the drain of the third MOS transistorat a node.
 12. The bias current circuit of claim 11, wherein the node isdirectly coupled to a resistor of the filter.
 13. The bias currentcircuit of claim 11, wherein the node is directly coupled to a capacitorof the filter.
 14. The bias current circuit of claim 13, furtherincluding a fifth MOS transistor and a sixth MOS transistor connected inseries between the input current and the load, the gate of the fifthtransistor connected to the gate of the third transistor; the gate ofthe sixth transistor short-circuited to the drain of the sixthtransistor and connected to the drain of the fifth transistor at thenode.
 15. The bias current circuit of claim 14, further including aseventh MOS transistor coupled to the drain of the second MOS transistorand the load and located in series with the oscillator; and a resistorof the filter is electrically connected between the gate of the fourthMOS transistor and the gate of the seventh MOS transistor.
 16. The biascircuit of claim 15, wherein the capacitor is electrically connectedbetween the node and the gate of the seventh MOS transistor.
 17. Thebias current circuit of claim 11, further including an additional MOStransistor extending intermediate of, and interconnecting, the secondMOS transistor and the oscillator.
 18. The bias current circuit of claim11, further including a variable resistor extending intermediate of, andinterconnecting, the second MOS transistor and the oscillator.
 19. Thebias current circuit of claim 11, wherein the drain of the second MOStransistor is connected to the capacitor of the filter.